Electronic equipment is susceptible to damage owing to high spike voltages being coupled to the A-C power mains from which they obtain operating energy. Such spike voltages can be caused by surges arising from inductive loads, lightning storms, or the like.
The highest potential transient spike voltages are short in duration, typically tens of microseconds. These high potential spikes can range into many thousands of volts, and many thousands of amperes. Such high voltages tend to overstress electronic components, subjecting them to hazardous and unwanted operating conditions. Owing to the low source impedance of the A-C mains and very high potentials involved, very large currents flow during component breakdown. As a result, dissipation in the components may be substantial, often resulting in their being destroyed.
Heretofore, it was customary to use power mains frequency isolation transformers in D-C power supplies obtaining energy from the A-C power mains, to isolate the power mains voltage from the equipment for safety reasons. The self-inductance and distributed capacitance of the isolation transformer windings integrated the energy contained in transients caused by lightning or other sources received from the A-C power mains. As a result, voltage excursions delivered to the load were minimal and were current limited by the transformer. More recently, to lower cost and reduce size and weight, there has been a tendency to eliminate mains isolation transformers. Instead, the A-C mains voltage frequency is rectified directly to supply direct operating voltage to a switched-mode voltage regulator. This regulator, in turn, provides electrical isolation via a high frequency transformer that supplies subsequent electronic equipment (such as a television receiver or computer equipment). This newer design (switched-mode supply) electronic equipment has proven to be undesirably susceptible to damage from surges or transients on the A-C power mains.
In response to this problem, a number of line surge suppressors have been marketed, though none to the knowledge of the present inventor, other than that disclosed in the parent of this application, has offered satisfactory protection for electronic equipment during lightning storms with reasonable life, cost and reliability. A number of available line surge suppressors contain voltage limiting devices (usually Metal Oxide Varistors--MOVs) for connection in shunt across the A-C power mains, which voltage limiting devices have very large ONE-TIME surge current handling capability--e.g. 4500 or 6500 amperes. Spark gaps, gas discharge tubes and zener diodes also have all been employed, but each such component has either a serious reliability problem or an operational characteristics problem, or both, as they are applied in power line surge suppressors (see, for example, U.S. Pat. Nos. 4,563,720--Clark, 3,793,535--Chowdhuri, 4,068,279--Byrnes, 4,463,406--Sirel, 4,628,394--Crosby et. al.).
A common problem with the shunt protector design concept is that the A-C mains have a very low source impedance, so the shunt voltage regulation afforded by these voltage limiting devices is severely compromised due to the lack of current limiting. Furthermore, the high current rating of 4500 or 6500 amperes of such varistors is a ONE TIME rating, after which the characteristics of the metal oxide varistor are permanently compromised for subsequent transients. What has been found to be needed to make the shunt voltage regulation of the limiting devices effective is a series pass element as described in my above-noted application to augment the impedance of the A-C power mains and the A-C line cord. Unless such a series-pass element is provided, a high energy transient applied directly across the varistor or other voltage limiting device is supplied from a very low source impedance that is capable of continuing to support current flow at unpredictably high currents, perhaps approaching or exceeding the limit of the surge current handling capability of the voltage limiting device, and compromising performance for subsequent transients. The thousand ampere or more surge current is accompanied by a several hundred volt peak voltage developed across the voltage limiting device, and the electronic equipment receiving operating voltage from across the voltage limiting device often succumbs to the overvoltage. This is so even though (as the manufacturers of the line surge suppressors point out) the line surge suppressor itself is capable of withstanding the rated lightning-caused surge at least once before becoming ineffectual. Furthermore, there is generally no indication given when the surge suppressor has experienced such an event and the transient protection of the varistor no longer meets the original specifications.
In order to provide the necessary protection, a series pass element is required between the A-C mains and the voltage limiter which is able to withstand 6,000 volts peak, which carries the line current and exhibits sufficiently low impedance that its insertion loss during normal operation at A-C power mains frequency is reasonably low, but exhibits high impedance and high loss during voltage spikes.
It has been found to be difficult to obtain consistently adequate shunt regulation against high energy surge voltages utilizing only one stage of voltage limiting. Furthermore, voltage limiting devices can survive high energy surges indefinitely only if the current is properly limited. For these reasons, a cascade of shunt voltage limiters has been found to be desirable to protect a load such as a switching regulator from these surges on a repeated basis.
In a cascade of voltage limiters, the first stage is required to withstand the greatest stress (particularly voltage). Filter capacitors in such a first stage necessarily are larger, have a higher voltage rating and therefore are more costly. For example, in the first stage limiter described in my above-identified application, a 6000 volt, 3000 ampere surge occurring on the A-C mains could result in a level of as much as 500 volts being produced across the storage capacitor in that first stage. A single electrolytic capacitor of the required capacitance and having a surge rating of 500 volts is costly and bulky. Furthermore, such a capacitor would exhibit a series resistance which would have to be taken into account in the circuit design, thereby requiring a higher valued capacitor than simple calculations would indicate. In order to reduce the cost and size of such components, it is desirable to provide an auxiliary energy storage means in a first stage voltage limiter, the auxiliary storage means being arranged to be operative only in response to surge energy above a predetermined level for storing momentary excess energy surges and thereafter slowly dissipating the energy so stored.